The present invention relates to a hydraulic brake system arranged to control both the brake slip and the traction slip.
A prior art device is disclosed in German patent application published without examination, No. 3,800,854.
The technical background on which the invention is based can be described as follows:
Basically, it is state of the art to prevent an excessive traction slip (i.e. racing of the wheels) by exerting a brake force onto the driven wheels. In this context, the brake torque counteracts the driving torque and reduces it to such an extent that the residual torque is transmitted by the frictional forces between the tires and the roadway.
The brake torque must be variable in order to be able to adjust an optimum traction slip. For this reason, it is proposed to transform those brake systems which provide a brake slip control in such a manner that they permit a traction slip control as well. A basic problem in this transformation is that, at the start of a traction slip control action, the hydraulic pressure fluid must be conducted into the brake circuits in order to build up a brake pressure which will subsequently be varied.
In the event of a brake slip control action, this hydraulic pressure fluid is displaced from the master cylinder into the brake circuits by actuating the master cylinder.
It is known from the prior art to connect the pump, which is provided in the device for the brake slip control, to a reservoir, so that, for the traction slip control, the pump will suck hydraulic pressure fluid from the reservoir and conduct it into the brake circuits. In the event of a traction slip control action at high vehicle speed, however, a sudden pressure build-up will be required which cannot be brought about by such systems without a pressure accumulator. In addition, the brake fluid will be highly viscous during cold weather, so that the pump cannot deliver its full capacity quickly enough.
It has, therefore, been proposed in German patent application published without examination, No. 3,803,363 to provide a high-pressure accumulator on the delivery side of the pump. As a rule, the high-pressure accumulator is dimensioned such that it puts at disposal a volume which is sufficient to fill the wheel brakes of the driven wheels and to generate a pressure in the wheel brakes which safely leads to the wheels failing to race. Depending on the specific brake system, this pressure amounts to approximately 100 bar or more. These prerequisites lead to the accumulator being very large and causing additional cost.
In contrast, it is proposed in the already mentioned German patent application published without examination, No. 3,800,854 to provide an accumulator on the suction side of the pump. This accumulator is actuated by a vacuum receptacle and can be considered a low-pressure accumulator because it brings hydraulic pressure fluid to the suction side of the pump at a comparatively low pressure level, which is then delivered by the latter into the brake circuit. The accumulator, therefore, does not put at disposal the required volume of pressure fluid. The pressure build-up is brought about by the pump. In this set-up, the speed of the pressure build-up depends again on the efficiency of the pump.
It is desirable, therefore, to provide a relatively inexpensive, space-saving brake system which will ensure a sufficiently rapid filling of the wheel cylinders for the traction slip control at high speeds.